1. Field of the Invention
The present invention relates to a high-pressure discharge lamp, and more particularly to the shape of an electrode of a high-pressure discharge lamp and a method of manufacturing such a high-pressure discharge lamp.
2. Description of the Related Art
In the field of ultrahigh-pressure mercury lamps (high-pressure discharge lamps), as electrodes are designed for shorter arcs across the electrode gap, reductions and fluctuations in the illuminance due to temperature rises of the electrodes and variations of discharge starting points are posing serious problems.
For suppressing a blackening phenomenon due to a scattering of tungsten, it has been proposed, as disclosed in Japanese laid-open patent publication No. 2000-299086, to construct an electrode to meet the condition of 1/50*R3≦ΔL≦⅕*R3 where R3 represents the diameter of a thicker coil of the double winding of the electrode and ΔL represents the distance between the tip end of the thicker coil and the tip end of the electrode, and also to ensure that the tip end of the electrode be fixed in position by being welded when energized after the lamp bulb is sealed.
Japanese laid-open patent publication No. 6-13029 discloses an electrode shaped such that the diameter (D) progressively increases from the base end to the tip end thereof and the electrode has a tip having a small diameter (d) on the face of the thicker tip end, the diameters satisfying the relationship of 0.2<d/D<0.5.
Japanese laid-open patent publication No. 10-92377 reveals an electrode having a structure of large thermal capacity disposed on the tip end thereof and having a diameter greater than the shank of the electrode, and a heat radiator disposed behind the electrode and comprising a tungsten wire coiled around the shank of the electrode.
Japanese laid-open patent publication No. 10-208693 shows an electrode having a thick wound portion which comprises a tungsten wire (diameter=φK) coiled as a double winding around an electrode rod (diameter=φE) from behind (d) the tip end of the electrode rod. If the lamp power is represented by W and the initial current peak value by A, then the ranges of 0.0017×W+0.18≦φE≦0.0017×W+0.38 and A×√d/(φE+φK)≧12 are satisfied.
Conventional high-pressure discharge lamps have suffered the problems of the blackening of outer casings due to a scattering of tungsten and illuminance fluctuations, and various countermeasures have been proposed to improve those shortcomings.
Japanese laid-open patent publication No. 2000-299086 attempts to avoid the blackening phenomenon by limiting an initial structure of the tip end of the electrode such that the distance between the electrodes remains unchanged even when they are melted by a discharge caused by an initial current. However, the disclosed electrode structure fails to avoid the instability of a discharge starting point which is responsible for illuminance fluctuations.
According to Japanese laid-open patent publication No. 6-13029, the electrode is prevented from being heated by a large current based on a large thermal capacity which is provided by a frustoconical electrode shape with a projection at its tip end. However, since the tip end of the electrode has a blocky large shape, the cooling factor is poorer than an electrode having a coiled heat radiator, and the electrode has a large size and is not sufficiently efficient.
According to Japanese laid-open patent publication No. 10-92377, the heat radiating coil is wound around the electrode, and the electrode shank at the tip end is melted into a semispherical electrode, so that the electrode can easily be machined. The electrode surface is generally semispherical in shape and may be machined into a truncated conical shape. However, the disclosed electrode fails to prevent the instability of a discharge starting point, and is expected to suffer illuminance fluctuations.
According to Japanese laid-open patent publication No. 6-13029, the electrode has an electrode portion greater in diameter than the electrode shank to suppress an excessive temperature increase with an electrode structure having a large thermal capacity and a heat radiating structure of a coiled winding. However, the electrode portion on the tip end is spherical or truncated conical in shape, causing an unstable discharge starting point which is responsible for illuminance fluctuations.
Japanese laid-open patent publication No. 10-208693 discloses that the maximum temperature of the electrode is lowered to provide good service life characteristics and initial characteristics by lowering an initial current peak value immediately after the lamp starts to be energized. However, because the tip end of the electrode has a cross-sectional shape which is equal to the cross-sectional shape of the electrode rod, the electrode causes an unstable discharge starting point which is responsible for illuminance fluctuations. Furthermore, there is a danger of a coil discharge between the tip end of the electrode and the tip end of the coil. Since it is important to provide conditions for setting up an allowable range for preventing the coil discharge, the disclosed electrode is disadvantageous in that a strict process of machining the electrode is required.